Laboratory Exercise Using Digital Images of
Specimens from the Peabody Museum
Developed from a workshop held at the
Yale Peabody Museum, July 2013
Sponsored by the National Science
Foundation and hosted by the Division of
Invertebrate Zoology in conjunction with
Peabody Museum Public Education
Morphological Variation in
the Asian Shore Crab,
Hemigrapsus sanguineus
General Data Gathering
• Distinguish between males and females by examining
the underside of the crabs
• Make carapace measures using the dorsal images of
the crabs
• Using the chelae (claw) images, measure “hand
height” and “hand length” of each
• Enter raw values into Excel; distinguish between
individual crabs by referencing catalog number
embedded in picture
Sex Determination in Crabs
• Examine the underside of a crab (image)
• Males have narrowed abdominal plate; the
sides are often somewhat concave; plate does
not cover entire underside
• Females have a broadly rounded abdominal
plate; lateral margins convex; plate covers
most or all of underside
Underside comparison of male
versus female crabs
Carapace measures, variable 1 and 2:
Carapace Length (CL) and Carapace Width (CW)
Chelae measures, variables 3 and 4:
Chelae Height (ChH) and Chelae Length (ChL)
Rules to remember
• To get accurate results one must be consistent
in measuring
• Strive for perpendicular measures
• Lengths and widths should reflect absolute
longest/widest distances
Measurement Techniques
• Use marks on paper held to screen image to
measure a distance
• Compare distance to scale bar; small
gradations are millimeters
• Record length to nearest 0.5 millimeter
• Most accurate measure will be from the edge
of one gradation to another (black edge to
black edge)
Measuring with pencil mark on paper held
against image on computer screen
Compare measurement to scale bar
(21 mm in this example)
Alternative measurement technique:
Calipers, reversed to hide numbers (avoids confusion)
Adjustable compass or dividers can also be used
for measuring
Data Capture
• Students, working in teams, should first enter raw data
manually on the data worksheet provided
• Data are next consolidated into an Excel spreadsheet, see next
panel for example
• Note that all measures are recorded individually, before the
ratios are derived; allows for auditing of data and results
• Consider highlighting in a different color
questionable/irregular variables such as indeterminable sex or
left chelae substitutions (when right was missing); allows for
isolation and removal from analysis if necessary
Worksheet for manual collection of data
Data Capture – Excel Worksheet 1
Analysis format – Excel Worksheet 2
Data Analysis dialog window
Data Analysis output worksheet
Data Analysis
• Simplest example will be to calculate the ratio of
chelae height versus chelae length
• Comparison will be made between males and
females, i.e. which has the more robust claw?
• Carapace length versus width is another simple
metric to measure and compare between males and
females – who is stouter?
• Refer to instructional document for details
Additional Considerations
• Many more variables can be added:
• Width between the eyes (orbital width)
• Length of major section of one or more legs (“femur
length” or second article)
• Dactyl length (i.e. moveable “finger”) on chelae
• Consider testing data without the left-hand claws –
does this make a difference?
Further Questions to Consider
• How would collecting more specimens affect
the analysis?
• What other comparisons may be useful, such
as temporal and geographical variables, and
what would the results potentially indicate?
• What would more robust claws in one sex
potentially indicate, and why?